1. Field of the Invention
The present invention relates generally to diving systems and more particularly to systems and apparatuses for controlling and monitoring rebreather systems and managing the use of dive resources.
2. Background Discussion
Self-contained breathing apparatuses used for underwater diving traditionally are categorized either as open circuit systems or rebreather systems. Open circuit systems are relatively simple and well understood in the art but are inefficient and typically require a large breathing gas supply to provide a reasonable dive time. Each breath inhaled by a diver from art open circuit system is exhaled to the surrounding environment, wasting any oxygen in the breathing gas that is not metabolized by the diver during the respiration cycle.
Rebreather systems recycle each breath exhaled by a diver by removing the carbon dioxide generated by the diver and replacing the oxygen consumed by the diver during the respiration cycle. Essential components of rebreather systems are a breathing loop comprising a diver's lungs, a mouthpiece, breathing gas supply, a solenoid or flow valve adapted to add breathing gas to the breathing loop, a pressure regulator for the breathing gas supply, a scrubber canister adapted to remove exhaled carbon dioxide, a counterlung, a valve adapted to vent or purge gas from the breathing loop if the total gas pressure in the breathing loop exceeds a preselected pressure value, flexible, gas-impermeable hoses connecting the various components and uni-directional check valves to control the flow of breathing gas through the loop. Rebreather systems may father be categorized as semi-closed circuit systems or closed circuit systems, with each type of rebreather system comprising additional suitable components. Examples of semi-closed circuit and closed circuit rebreather breathing loops are shown in, for example, U.S. Pat. Nos. 5,503,145 and 6,302,106.
In a semi-closed circuit system known in the art, the breathing gas supply commonly comprises one high pressure cylinder containing an oxygen-enriched gas mixture, which typically is introduced to the breathing loop at a preselected, constant flowrate to replace oxygen consumed during a dive.
In a closed circuit system known in the art, the breathing gas supply commonly comprises one high pressure cylinder containing pure oxygen. A closed circuit system known in the art would additionally typically comprise one or more oxygen sensors adapted to measure the partial pressure of oxygen in the breathing loop during a dive and a computer processor adapted to control the solenoid for the purpose of adding oxygen to the breathing loop as needed to maintain the oxygen partial pressure above a minimum, viable value. A closed circuit system also commonly comprises a high pressure cylinder containing an inert gas or mixture of inert gases, also referred to as diluent gases, that may be added to the breathing loop via a solenoid and pressure regulator to prevent the counterlung from collapsing due to increasing ambient pressure.
A primary consideration in the design and use of semi-closed and closed circuit rebreather systems is the minimization of the risks of hypoxia, in which the diver is deprived of a life-sustaining oxygen supply, and hyperoxia, which occurs when the diver breathes unsafe elevated oxygen levels. Hypoxia can render a diver unconscious and cause drowning. Hyperoxia may lead to oxygen toxicity, which can have severe physiological effects that can lead to the death of the diver. Oxygen toxicity can manifest as either central nervous system (CNS) oxygen toxicity or pulmonary oxygen toxicity.
Hypoxia and hyperoxia are understood as depending on the partial pressure of oxygen in the breathing gas loop. The partial pressure of oxygen is equal to the product of the total pressure of the gas mixture and the concentration, or fraction, of oxygen in the gas mixture, also expressed as PPO2=Ptotal×FO2. Total gas pressure in the breathing loop increases with ambient pressure, which increases by one atmosphere, or bar, per each ten meters of depth. Accordingly, at a constant oxygen concentration, the partial pressure of gas in the breathing loop increases as depth increases and decreases as depth decreases.
Hypoxia occurs when the partial pressure of oxygen in the breathing loop is less than 0.21 bar, which is the ambient partial pressure of oxygen in the atmosphere at sea level. The minimum life-sustaining value of partial pressure of oxygen is 0.16 bar. Maintaining the partial pressure of oxygen in the breathing loop above 0.21 bar will minimize decompression time for the diver. However, pulmonary oxygen toxicity can result from prolonged exposure to oxygen partial pressures above approximately 0.5 bar, and CNS oxygen toxicity becomes a significant risk when the partial pressure of oxygen in the breathing loop is greater than 1.6 bar.
In semi-closed circuit systems, the oxygen-enriched gas mixture is usually added to the breathing loop at a constant rate selected to maintain the partial pressure of oxygen in the breathing loop between 0.21 bar and 1.6 bar based on the estimated oxygen consumption profile of the diver during a dive. However, the rate of addition typically is not automatically adjusted during a dive in response to the actual partial pressure of oxygen in the breathing loop, even though a diver's rate of oxygen consumption, and therefore the oxygen concentration in the breathing loop, may deviate considerably from the rate of consumption estimated prior to the dive. Inequality between the rate of addition and rate of consumption of oxygen can result in depletion or accumulation of oxygen in the breathing loop. Therefore, transient states may occur and persist in semi-closed systems during which the breathing loop contains either hypoxic (i.e. less than 0.21 bar) or hyperoxic (i.e. greater than 1.6 bar) oxygen levels.
Closed circuit rebreather systems provide monitoring and adjustable control of the partial pressure of oxygen in the rebreather breathing loop during a dive. In recognition of the safety and performance concerns outlined above, closed circuit rebreather systems will usually be configured to maintain the partial pressure of oxygen in the breathing loop at a preselected value between 0.21 bar and 1.6 bar for the purpose of reducing the risk of hypoxia and hyperoxia. However, the sensors and automated control systems in closed circuit systems are susceptible to malfunctions, which can lead to hypoxic or hyperoxic oxygen levels in the breathing loop. Additionally, the use of pure oxygen in closed circuit systems creates handling and cleanliness issues with regard to the breathing gas supply.
A further disadvantage of semi-closed circuit and closed circuit systems is that excessive venting of gas from the breathing loop may occur. Semi-closed circuit systems are inherently inefficient because the fixed rate of addition of the oxygen-enriched gas mixture to the breathing loop may exceed the diver's rate of oxygen consumption and cause continual venting or purging of gas from the breathing loop to maintain total gas pressure in the breathing loop below a threshold value.
In closed circuit systems, excessive venting may occur during the ascent phase, during which ambient pressure, and correspondingly total gas pressure and partial pressure of oxygen in the breathing loop, decrease. For example, if the preselected control value for oxygen partial pressure in the breathing loop is set close to 1.6 bar for the purpose of minimizing the diver's decompression time, the closed circuit rebreather control system will increase the addition of oxygen to the breathing loop to compensate for the decrease in partial pressure of oxygen that results from the decrease in ambient pressure. However, the increased volume of gas added to the breathing loop will be vented to maintain total gas pressure in the breathing loop below a threshold value.
The rate of venting of unutilized breathing gas from semi-closed circuit and closed circuit systems may be less than typically experienced with an open circuit system but may nonetheless be sufficient to reduce total dive time or necessitate the use of a larger gas supply to accommodate decompression time during ascent. Additionally, the venting of gas from the breathing loop creates bubbles that may, for example, startle marine life that the diver is attempting to observe, visibly indicate the presence of the diver to observers on the surface or have other undesirable effects.
In view of the limitations of rebreather systems brown in the art, a need exists for a rebreather system that provides adjustable control over the partial pressure of oxygen in the breathing loop as well as control over the volume of gas in the breathing loop, and therefore the rate of gas venting from the breathing loop. Such a system would also desirably be free of the disadvantages of using pure oxygen as a breathing gas supply.
Dive monitoring systems known in the art continuously measure, calculate and display parameters such as remaining gas supply, decompression time, partial pressure of oxygen in the breathing loop and depth during a dive to keep a diver informed of the dive profile. These devices are typically configured to display the current status or value of one or more parameters corresponding to dive resources and may also be adapted to assist the diver in pre-planning the dive based on end-of-dive requirements. There is a need for a dive monitoring system configured to continuously inform the diver of remaining dive time based on current dive resources and their usage, inform the diver of the dive resources required based on the dive plan specified by the diver and automatically adjust partial pressure of oxygen in the breathing loop as needed to make it possible for a diver to achieve a target dive time.